Mobile machinery, i.e., machinery capable of self=propelled movement across a floor or earthen surface, is available in a wide variety of configurations. Each is designed for a particular purpose or for a few related purposes such as material handling or earthmoving and digging. Machines of the latter type, often referred as "construction machinery," include backhoes, front end loaders and "hybrids," loader-backhoes.
Advantageously, the working implements of backhoes, front end loaders and hybrid machines are powered by a hydraulic system controlled by lever-like handles mounted in the operator's compartment. Such handles actuate hydraulic valves which operate hydraulic cylinders and, frequently, rotary motors to power auxiliary equipment.
A front end loader is a machine mounted on crawler tracks or rubber tires and having a scoop-like bucket mounted on a pair of boom-like arms attached to the front of the machine. The height and attitude of the bucket are controlled by the operator to move material from one place to another, e.g., to remove snow from a pile and place it into a haulage truck. Of course, many other types of material moving and digging tasks are possible with a front end loader.
A backhoe (which may also be crawler or rubber mounted) has a bucket mounted at the end of an articulated arm. Such arm has inner and outer stick-like "elements" known as the boom and the dipper, respectively.
(One may closely approximate the boom, dipper and bucket by extending an arm--albeit somewhat uncomfortably--to an "elbow-up" and "wrist-up" attitude with the hand cupped, analogous to the bucket, and the fingers facing rearward, analogous to the bucket teeth. The shoulder represents the boom connection to the machine and the backhoe articulated arm "pivot points" correspond to the shoulder, the elbow and the wrist.)
The boom, dipper and bucket are mounted at the rear of the machine. The operator faces forward when moving the machine from place to place and rearward when operating the backhoe. Backhoes are frequently used for such tasks as digging trenches for water pipes or the like.
Inevitably, a hybrid machine emerged which has both a front end loader and a rear-mounted backhoe. Since the loader function is nearly always used when moving the machine and since the backhoe function is nearly always used (or should be used) only when the machine is stationary and braced by outrigger-like stabilizers, this was a natural evolution.
At least early as the mid-1960's, backhoes were developed in which the dipper--analogous to the human forearm--is extensible or telescoping. A telescoping dipper permits the operator to dig more deeply, reach further to unload the bucket and reach over obstacles, to name but a few advantages. Exemplary backhoes with extensible dippers are depicted in U.S. Pat. Nos. 3,298,548 (Long et al.) and 3,624,785 (Wilson).
U.S. Pat. No. 5,313,795 (Dunn) shows a circuit for use on a hydraulic backhoe while U.S. Pat. No. 4,966,066 (Kauss et al.) shows a two-priority-valve circuit used with a utility vehicle pulling a trailer. Such circuit uses a variable-displacement pump with a pressure/flow compensator and prioritizes steering, trailer brakes and auxiliary devices in that order.
And backhoes may be equipped with "attachment" valves to which auxiliary hydraulic tools may be connected for operation from the machine hydraulic system. Such tools may be of the "high flow" fully mounted type or of the "low flow" hand-operated type. An example of the former is a large auger or pavement breaker mechanically mounted on the outer end of the dipper in place of the bucket. Such tools require relatively high hydraulic fluid flow rates, e.g., 20-30 gallons/minute (GPM), and are operated using what may be referred to as a mounted attachment valve. ("Mounted" refers to the tool, not the valve which is always mounted on the machine.)
On the other hand, hydraulically-powered, hand-operated tools, e.g., jackhammers and the like, are attached to the machine only by hydraulic hoses coupled to what may be referred to as a hand-held attachment valve. Such hand-operated tools may require flow rates in the range of 3 to 12 GPM or so. (As with the term "mounted," the term "hand-held" refers to the tool, not the valve controlling the tool.)
While backhoes (with or without extensible dippers) have been used mostly for digging, what may be described as a phenomenon has become evident. That is, operators of such machines are using the backhoes not only for digging but also as a "fine grading" implement. In so doing, the operator moves the bucket teeth across the ground or along a vertical or angled surface in a way to smooth such surface and make it generally flat.
Even if the backhoe has no extensible dipper, performing fine grading with a backhoe is a tricky task and requires very close control by the operator. This is due in large part to the triangular shape generally defined by the boom and dipper during fine grading. To keep the bucket teeth moving in a generally straight-line path, the boom, dipper and bucket positions must all be controlled simultaneously and accurately. And the inclusion of an extensible dipper adds another level of complexity to the control task. A leading manufacturer of machines of the foregoing types is Case Corporation of Racine, Wis.
While machines of the foregoing types have been generally satisfactory for their intended purpose, they tend to be characterized by certain disadvantages. Some of these disadvantages arise from emerging new patterns of use.
For example, operating the conventional backhoe valves to manipulate the swing, boom, dipper and bucket may "starve" the extend valve and its circuit used for extending and, more particularly, retracting the dipper during fine grading. Dipper retraction thus becomes erratic and the surface to be graded is made undesirably uneven.
Yet another disadvantage relates to the fact that most backhoes are equipped with two hydraulic pumps, the output flow rates of which may differ from one another by a ratio as high as 2:1 or more. To cite an example, a backhoe may have a 12 GPM pump and a 24 GPM pump, both flow rates being at rated engine speed.
In known arrangements involving attached hand-held tools, it is common practice to power such tools from the smaller of the two pumps while operating the engine at about rated speed. However, this means that the output flow from the larger pump (like that from the smaller pump) also circulates through the hydraulic system.
Inevitably, hydraulic systems having oil circulating therein evidence "pressure drop" as the oil moves through piping and valves and otherwise around the system. By a known equation, pressure drop and flow rate relate to horsepower and in such circulating systems, these horsepower losses (which are manifested as unwanted heat in the hydraulic system) are known as "parasitic losses." Such losses translate into higher (and with the advent of the invention, unnecessary) operating costs, engine wear, exhaust fumes and the like.
And excess heat manifested in high oil temperature is undesirable for a more subtle reason. The temperature of the hand-held tool, being hydraulically operated, tends to rise toward that of the hydraulic oil passing through it. At some point, the tool may be too hot to handle, at least comfortably.
An improved hydraulic system which addresses such disadvantages would be an important advance in the art.